Because foliage instances are rendered as clusters in a single draw call, each cluster is either rendered or not rendered based on occlusion. If you set a value in the End Cull Distance parameter, the clusters will also be culled beyond that distance. However, this will cause groups of meshes to disappear abruptly as the entire cluster goes out of range.
This can be reduced by adding a Start Cull Distance parameter and then setting up the Material appropriately.
In the vertex shader, a per-instance fading factor is calculated, which goes from 1.0 at the start distance to 0.0 at the end distance.
This is accessible in the Material using a PerInstanceFadeAmount node.
If you connect that to an opacity or masking value, you can use it to fade instances over a distance before they reach the Cull Distance End and are removed from rendering.
The example Material below multiplies the Material mask by the fading factor to thin out the foliage mesh's leaves until they completely disappear.